Formation of clay minerals under hydrothermal influence is the result of rock alteration by circulating hot water in the Earth’s crust. A pre-existing rock-forming mineral assemblage is altered to a new set of minerals which are more stable under the hydrothermal conditions of temperature, pressure, and fluid composition. The interaction of hot water and rocks forms a spatially and temporally regular zonal pattern of new clay minerals, as the fluid with cooling temperature moves through the surrounding rock mass. This chapter discusses the formation of clay minerals in such dynamic processes of hydrothermal alteration. The approach is one of clay-mineral facies formed under conditions of massive alteration in the rocks. The chemical and mineralogical changes which occur on the scale of a rock or rock mass are considered to have been dealt with in the preceding chapter. The exact process of change via local, vein-influenced exchange processes is ignored for simplicity (see Chap. 6).

Formation of Clay Minerals in Hydrothermal Environments

A complete conversion series for mica/smectites was found in a hydrothermal alteration envelope around Kuroko-type ore deposits at the Shinzan area, Akita Prefecture, Northeast Japan. The minerals are an alteration product of volcanic glass of Miocene age and are commonly associated with zeolites and silica minerals. Degrees of ordering of interstratification of the minerals change discontinuously from Reichweite g = 0 (100–55% expandable layers) to g = 1 (45–20% expandable layers), and from g = 1 to g = 2 (<20% expandable layers). This pattern of conversion differs from the behavior of mica/smectites during burial diagenesis which undergo a continuous change in ordering type, and from the behavior of rectorite which displays a constant expandability and ordering (45–55%) over a wide range of conditions. Differences between these minerals were also found in the relationships between expandability and total layer charge, and between expandability and number of non-exchangeable interlayer cations. In mica/smectites from the Shinzan area, chemical changes in the interlayers and tetrahedral and octahedral sites are consistent with a reaction in which K-enrichment and K-fixation in the interlayers are controlled by an increase in negative layer charge. This conversion occurred in response to a steep geothermal gradient and migrating hydrothermal solutions.

Further investigations of a conversion series of dioctahedral mica/smectites in the shinzan hydrothermal alteration area, northeast japan

. The Rodnikovoe gold deposit situated in a presently active hydrothermal system located north of the Mutnovsko-Asachinskaya geothermal area in southern Kamchatka, Far Eastern Russia, consists of typical low-sulfidation quartz-adularia veins in a host rock of diorite. The age of the mineralization was dated by the K-Ar method as 0.9 to 1.1 Ma based on adular-ia collected from the veins. Representative ore minerals in the deposit are electrum, argentite, aguilarite, polybasite, pearceite and lenaite. Dominant alteration minerals are adularia, α-cristobalite, chlorite, illite and kaolinite. Hydrothermal solutions of neutral pH were responsible for the mineralization, which is divided into six stages defined by tectonic boundaries. Gold mineralization occurred in stages I and III. Hydrothermal brecciation occurred during stages III, IV and VI. Stages II, IV, V and VI were barren. The estimated ore formation temperature based on a fluid inclusion study is 150 to 250 °C at a depth of approximately 170 m below the paleo-water table. Boiling of hydrothermal fluids is hypothesized as the cause of the intermittent deposition of gold ore. The sulfur and oxygen fugacities during the deposition of anhydrite prior to the hydrothermal brecciation were higher than those during the gold mineralization stages. The occurrence in the hydrothermal breccia of fragments of high grade Au-Ag and polymetallic ores suggests that higher grade mineralization of these metal ores might have occurred in a deeper portion of the deposit.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1751-3928.2002.tb00146.x

Hydrothermal Gold Mineralization at the Rodnikovoe Deposit in South Kamchatka, Russia

Two distinct zonal sequences of clay minerals (H- and L-type) were found around silicified rocks in the Makurazaki volcanic area, Kagoshima Prefecture, Japan. The clay mineral sequences from the inner to the outer parts of the alteration aureoles are: 1. H-type, pyrophyllite → dickite → 2M2 mica → sudoite → tosudite; and 2. L-type, kaolinite → rectorite → smectite. The structural formula for the sudoite is: (Al1.04Mg1.28Fe3+0.20Ti0.03Li0.01K0.02Na0.01(OH)6Al2.00(Si3.54Al0.46)O10(OH)2. It is characterized by relatively large amounts of Mg and very small amounts of Li. The polytype is identified as IIb. The chemical analysis of tosudite shows that the sample is composed of an interstratification of sudoite-like and beidellite-like layers. The structural formula for rectorite is: (K0.45Na0.19Ca0.01Mg0.01)(Al1.81Fe3+0.04Mg0.13Ti0.03)(Si3.41Al0.59)O10(OH)2, suggesting that the nonexpandable and expandable layers have K-mica-like and beidellite-like compositions, respectively. These clay minerals in the H- and L-type alteration aureoles were formed under relatively high- and low-temperature conditions, respectively, with pH value and K- and Mg-activities increasing as the fluids ascended through the wall rocks.

Mineralogy and genesis of clays in postmagmatic alteration zones, makurazaki volcanic area, kagoshima prefecture, japan

Two specimens of interstratified ammonium-bearing mica/smectite have occurred at Aira, Kagoshima Prefecture, Japan. The mineralogical properties of the specimens were examined by chemical analysis, X-ray diffraction, thermal analysis, infrared absorption, electron diffraction and electron microscopic observation. The ammonium contents of the specimens are 0.84 and 1.45wt%, respectively. The structure of interstratification is expressed by Reichweite g = 1 having about 17 and 31% expandable layers. The IR spectra of the specimens exhibit pronounced absorption bands of ammonium ions at 1428, 2820 and 3010cm−1. The morphologies of the specimens are characterized by lath- and flake-like habits, respectively.

Shoji Higashi

Papers

Mineralogical studies of hydrothermal dioctahedral mica minerals